The principle of this invention is that the fluid in the lower part of the cavities is vaporized by heat conducted through the lower surface of the heat transfer device. The vapor which moves up to the upper part of the cavities is then condensed by cooling at the upper surface of the heat transfer device and consequently the latent heat of vaporization is transferred to the outside through the upper surface of the cavities.
Heat is, therefore, transferred from the lower surface of the cavities by the evaporation of the working fluid to the upper surface of the cavities by the condensation of the vapor in the presence of gravity. Since heat can not be transferred from the upper surface of the heat transfer device to the lower surface of the device, it can act as a thermal diode.
When the fluid is condensed, it flows down along the walls of the cavities to the lower part of the cavities in the presence of a force field such as gravity because the density of the liquid phase of the working fluid is heavier than that of the vapor phase of the working fluid.
Therefore, when the plate type heat transfer device produced according to the present invention is installed in a place, heat is transferred only from the lower side to the upper side of the device and can not be transferred effectively in the reverse direction.
The heat transfer mechanism in the present invention employing the latent heat of vaporization of a working fluid is similar to that of the thermosyphon as described in U.S. Pat. No. 2,350,348.
However, because the structure of the heat transfer device by the present invention is quite different from that of the thermosyphon, the heat transfer route is different from that of the conventional thermosyphon. Conventional thermosyphons are made from tubings which have a relatively small area in the direction of heat flow due to the small diameter of the tubings used. In such conventional thermosyphons, heat is transferred into the tube through the side wall of the lower part of the tube containing the fluid and heat is removed through the side wall of the condensing part located in the upper part of the thermosyphon. Therefore, the conventional thermosyphon has disadvantages if it is to be applied to the situation where the area of heat transfer or the area which has to be maintained at a uniform temperature is relatively large in comparison to the area of the thermosyphon in the direction of heat flow.